Put Heptapod Workhorse in the Docker image

Since Heptapod Workhorse will be mandatory in Heptapod 0.13, this is necessary.

At this point, it won't be useful to have a separate Docker image for Heptapod Workhorse, because communication with the hgserve backend must still be local. Still we can use the multistage build patter, and later on decide to push octobus/heptapod-workhorse separately.